1. Field of the Invention
This invention relates to a method and an apparatus for core alignment of two optical fibers to be spliced or connected.
2. Description of the Prior Art
Because of their wide band and low loss, optical fibers can be advantageously used as transmission medium in a non-repeated long-haul transmission system covering a distance of the order of some tens of kilometers. At present, the optical fibers which suit actual service generally have a maximum length of about 10 kilometers beause of the restriction associated with equipment available for optical fiber manufacture. To prepare continuous optical fibers extending over the distance of some tens of kilometers, therefore, it becomes necessary to splice currently available optical fibers end to end. The connection of optical fibers is also necessary since optical fibers, when severed accidentally or otherwise, must be spliced together.
When connecting two optical fibers, efforts should be made to repress possible loss due to the work of connection as much as possible to ensure full utility of the characteristic low loss inherent in optical fibers. The degree of the loss due to the connection to two optical fibers is essentially determined by the amount of deviation (misalignment) between the centers of the cores of the optical fibers at their spliced ends. To obtain desired connection of two optical fibers with very low loss, therefore, it becomes necessary to minimize the amount of misalignment between the connected core ends of the optical fibers. The particular work involved for this purpose is generally referred to as "core alignment."
The principle which underlies the conventional method for core alignment is depicted in FIG. 1. One of the two optical fibers 11 to be connected is fastened to a stationary stage 13 and the other optical fiber 12 is fastened to a stage 14 finely movable in the directions x, y, and z as illustrated. The output light from a light source 15 is injected into the fiber 11. The incident light passes through the fiber 11 and the fiber 12 and reaches the optical detector 16. Then, required core alignment is effected by monitoring the optical power transmitted to the optical detector 16 and, at the same time, moving the stage 14 to a point at which the optical power displayed on the optical detector 16 reaches its maximum. This method is referred to as the method of monitoring be means of transmitted optical power.
This method, however, suffers from two major drawbacks.
One of the drawbacks is that a place for connection (a position at which the stages 13 and 14 are located), a place for incidence of light (a position at which the light source 15 is located), and a place for reception of the transmitted light (a position at which the optical detector 16 is located) are different. When cables using optical fibers as transmission medium are sequentially connected and laid over a great distance or when such cables have sustained breakage, for example, there is a possibility that the actual spots for the connection of optical fibers will be separated by tens to some hundreds of kilometers from the repeater station. In this case, the method entails grave inconvenience in that the transmitted optical power is monitored at the repeater station and the actual core alignment for the connection of optical fibers is carried out at a distant field. Ideally this method ought to be modified so that the core alignment at the site of the connection of optical fibers may be effectively carried out based on the information obtainable in the field (or at the site) of the core alignment. The aforementioned conventional method is incapable of any such modification.
The other drawback is that there are cases where the method of monitoring by means of transmitted optical power cannot be put to effect. In the case of a repeated setup incorporating a plurality of repeaters, the situation in which all the repeaters are operated and the optical power transmitted through them all is monitored is not improbable. In that case, the output power is fixed at a constant level by the automatic gain controllers (AGC) in the repeaters, irrespectively of the level of the input optical power to the repeaters. Consequently, the level of the optical power to the optical detector 16 does not include the information on the state of connection. Thus, the method of monitoring by means of the transmitted optical power can no longer be effectively adopted.